1. Field of the Invention
The present invention relates to a method for forming an isolation region in a semiconductor device, and more particularly, to a method for forming an isolation region in a semiconductor device, suitable to effect electrical isolation between adjacent photodiodes in an image sensor.
2. Discussion of the Related Art
A highly integrated semiconductor device, e.g., a CMOS image sensor, can be used to convert an optical signal into an electrical signal output using an array of active devices, e.g., photodiodes, for forming an optical image based on an incident light signal. Arrays arranged to be increasingly more dense result in increased device integration, that is, the number of pixel units on a single substrate or the number of photodiodes per pixel is increased. Thus, the distance between photodiodes is reduced. Shallow trench isolation (STI) technology is used to minimize the negative effects of such integration, such as, electrical interference experienced by adjacent photodiodes. FIGS. 1A-1C show a conventional method for forming an isolation region in a semiconductor device using STI technology.
Referring to FIG. 1A, a trench region T is formed by selectively etching a substrate 11, which undergoes thermal oxidation. As shown in FIG. 1B, the thermal oxidation results in the formation of a thermal oxide film 12 on an inner sidewall of the trench region T. Thereafter, a tetra-ethyl-ortho-silicate is deposited over the entire surface of the substrate 11 and is then substantially planarized to leave a tetra-ethyl-ortho-silicate film 13 before undergoing a high-temperature treatment to increase its density. The high-temperature treatment produces an isolation region as shown in FIG. 1C, in which a densified tetra-ethyl-ortho-silicate film 13′ fills the trench region T. The photodiodes (not shown) of a unit pixel are formed between isolation regions of the substrate 11.
The above-mentioned high-temperature processes, however, reduce a pinning effect in the photodiodes, which are to be formed between isolation regions of the substrate 11. The pinning effect is reduced due to diffusion of impurity ions in the substrate. In addition, interface defects occurring at a junction of the substrate 11 and isolation region degrade the electrical characteristics of the photodiode by increasing leakage current.